Weird blotchy shadows in blender cycles (denoising)
This is how to use Intel Optix Denoise node in compositor.
Basically instead of using the Blender denoiser, using this node allows you to denoise a scene using low samples, and without creating splotchy/dark areas.
Downsides? This node may cause blurring in some areas, when used with very low samples.
This image is with regular denoising, rendered at 4 samples (to show effect):

This is the same scene, but using the denoise node in the compositor to denoise:
This is the original image without any denoising:

As you can see, it has a major effect on render times, and denoises well, but it can be blurry on very low quality renders.
You can enable this node by going to the compositor, and clicking &quot;use nodes&quot;. Then add the denoise node between the image node and the output node. (You can press Shft+A to pull up a search menu for the node).
A simple connection between the image points should do the trick:

If that is too smooth, you can try connecting the image node to both the image and albedo points on the denoise node:
The node runs on your CPU, so there should be no reason why you can't get it to work.
For more info about connecting the denoise node, see this post:
How to properly connect the Denoise node in Compositor?